1. Field of the Invention
This invention relates to a thermodevelopable photographic material, and particularly to a thermodevelopable photographic material containing a compound which inhibits fog caused by heating (development) (to be referred to hereinafter simply as "fog").
2. Description of the Prior Art
Photographic processes utilizing silver halides have previously gained most widespread acceptance because they give rise to superior photographic characteristics, such as sensitivity and gradation, to methods such as electrophotographic or diazo-type photographic processes. Silver halide photographic materials used in these methods are first developed with a developer after imagewise exposure. Then, in order for the developed image not to discolor or fade under ordinary light illumination, or for the undeveloped part (to be referred to hereinafter as "background") not to turn black, the materials must be subjected to treatments such as stopping, fixation, rinsing or stabilization. Accordingly, the processing of the photographic material is time-consuming and laborious. Moreover, the handling of various chemical involves risks to the human body, and also the contamination of the processing rooms or the hands or garments of the working personnel. It is therefore very desirable to improve photographic processes which use silver halides so that the processing of the photographic material can be performed in the dry state without using a wet treatment, and the processed image is maintained stable.
Various efforts have been made to achieve this improvement. One of these efforts has resulted in a photographic process using a thermodevelopable photographic material comprising as a photographic element a major amount of a silver salt of a long-chain aliphatic carboxylic acid, such as silver behenate, saccharin.silver, or benzothiazo.silver, and a catalytic amount of a silver halide, as disclosed in U.S. Pat. Nos. 3,152,904; 3,457,075; 3,635,719; 3,645,739 and 3,756,829 and Japanese Patent Publication No. 22,185/70.
When, for example, a photographic material comprising a composition consisting of a fatty acid-silver salt, a reducing agent and a catalytic amount of a silver halide, as one of the thermodevelopable photographic materials heretofore proposed, is heated, an oxidation-reduction reaction between the fatty acid-silver salt and the reducing agent occurs due to the catalytic action of the silver halide at the exposed portion to form a silver image. However, at the same time, an oxidation-reduction reaction proceeds also at the non-exposed portion to cause undesirable fog.
In an attempt to prevent fog formation, a method has already been proposed in which a compound capable of donating a mercury ion is used. For example, U.S. Pat. No. 3,589,903 discloses that a thermodevelopable material having a reduced tendency to fog formation can be obtained by incorporating a compound capable of donating a mercury ion into a composition consisting of an organic silver salt, a reducing agent and a catalytic amount of a photographic silver halide.
As is well known, however, mercury compounds are toxic, and the photographic processing operation could be very risky. Furthermore, if unwanted coating solution containing mercury ions are discharged into rivers or the like, the mercury builds up in the fish and shell-fish, and will cause serious problems upon human consumption. Also when photographic materials containing mercury compounds are re-used, for example, as regenerated paper, the mercury compounds leach out to cause similar problems.
Accordingly, an object of this invention is to provide a thermodevelopable photographic material containing a compound which is not toxic, in contrast to mercury compounds, but which possesses a fog inhibiting effect as is obtained with mercury compounds.
Another object of this invention is to provide a thermodevelopable photographic material whereby the image obtained after development has reduced discoloration by light.
Still another object of this invention is to provide a thermodevelopable photographic material which can give good quality images free from fog and having high image density and good contrast.